Page 265 - Biodegradable Polyesters

10.2 Polylactide Strengthening and Strategies 243

mechanical properties [46]. As far as toughness of PLA is concerned, PLA can
be readily impact-modified or plasticized via melt-blending approaches [41, 45,
47]. Plasticization is frequently used to improve the processing behavior and the
flexibility of PLA-based materials [48]. The extent of plasticization depends on the
miscibility of plasticizers with host polymers as a function of molecular weight
and loading level [41], but undesirable decrease in stiffness of the as-plasticized
materials, together with the undesired leaching of the plasticizers over time are
often observed [41, 45, 47].
Blending PLA with immiscible rubber-like polymers represents a more inter-
esting way to reduce PLA brittleness, while keeping stiffness acceptable and pre-
venting any undesirable aging. This yields a new type of polymeric materials with
different properties, in which each polymeric partner provides its own feature in
terms of impact-absorbing ability from impact modifiers and stiffness from PLA
[49–51]. Recently, NatureWorks has defined the key parameters of impact modi-
fiers useful for toughening PLA as follows [52]:
• To be immiscible, to a certain extent, with the matrix
• To have high molecular weight (to avoid undesired leaching)
• To be distributed as small domains (usually 0.1–1.0 μm) in the matrix
• To have a good interfacial adhesion with the PLA matrix
∘
• To have a T at least 20 C lower than the use temperature
g
• To be thermally stable to PLA processing temperatures.
In addition to these strategies, the control of crystallization extent within PLA-
based materials can be another way to enhance the impact strength of PLA. The
change of crystalline morphology can affect the stress upon impact, and therefore
reduce the energy propagation on impact. Our newest review “Recent advances in
high performance poly(lactide): From green plasticization to super-tough materi-
als via (reactive) compounding” reports exhaustive examples in the field of rubber-
toughened PLA [47]. In contrast, the following paragraphs will merely emphasize
the rubber toughening of PLA upon the addition of impact modifiers as well as its
combination with nanoparticles, the formation of interpenetrated networks, and
annealing process/crystallinity extent as discussed in the following. This contri-
bution will first address the rubber-toughening and related mechanisms in corre-
lation with these strategies.

10.2.1
Impact and Toughening Mechanisms: General Considerations

The impact resistance of a material is its ability to withstand the application of
a sudden load without failure by dissipation of the impact stress energy. There
are two general failure modes, namely, “brittle fracture” and “ductile fracture.”
While brittle fracture is characterized by a relatively low energy dissipation and
no significant deformation, ductile fracture is characterized by a high energy dis-
sipation and a large-scale deformation [53]. A brittle–ductile transition is defined
as the point at which the fracture energy increases significantly with a mode of